Fire alarm system



NOV. 9,'1954 HAGEN 2,694,148

FIRE ALARM SYSTEM Filed Dec. 27, 1949 INVENTOR.

JERRY HAG EN United States Patent FIRE ALARM SYSTEM Jerry Hagen,Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company,Minneapolis, Minn., a corporation of Delaware Application December 27,1949, Serial No. 135,169

16 Claims. (Cl. 250-36) This invention relates generally to fire alarmsystems and is more particularly concerned with the type of fire alarmsystems which use an oscillator circuit which is affected by thepresence of undue heat.

Fire alarm systems have been known and used for many years. Some systemsuse fusible wires which are strung around a room with the wires meltingto break an electric circuit when the temperature becomes too great.Other types of systems use a temperature sensitive switch which may bemade to either open or close when the temperature becomes greater thanthat for which the switch is set. The present invention is concernedwith the second type of the above-mentioned systems.

Many existing fire alarm systems are bulky, the wiring of the detectorsis complicated, and special wiring is often required. The morecomplicated devices are subect to more malfunctioning and are lessdependable. It is therefore desirable to have a simple device which hasas few parts as possible which might cause a malfunction. The presentinvention uses a simple detector circuit and utilizes existingcommercial power wiring to make the connection between the detector andan amplifying device. In the present invention both the detecting deviceand the amplifying device may be plugged into ordinary wall sockets.

It is therefore an object of the invention to design a simple new andimproved, inexpensive fire alarm system.

Another object of the invention is to design a fire detection systemusing existing power wiring.

Another object of the invention is to design a fire alarm system usingan oscillator circuit having inductance coils in the plate and gridcircuits of an oscillator circuit which are loaded by a third inductancecoil.

Another object of the invention is to design a fire alarm system usingan oscillator circuit in which the oscillator circuit is normally soloaded as to prevent oscillation.

Another object of the invention is to design a fire alarfm system which,in the event of failure, will fail sa e.

Another object of the invention is to design a fire alarm system usingan oscillator circuit in which the load on the oscillator circuit isreduced sufiiciently to throw the circuit into oscillation upon a sensedtemperature reaching a predetermined value.

The invention generally comprises an oscillator circuit loaded by theimpedance of two power lines with the loading normally being sufiicientto prevent an alarm circuit from being operated. A temperature sensitiveswitch is placed across the two lines near the oscillator circuit andreduces the loading of the circuit, when the switch is closed, to causethe alarm circuit to be operated.

For a better understanding of the invention reference is made to thefollowing, more detailed description taken in conjunction with thedrawing in which:

Figure 1 shows a fire alarm system with the temperature sensitive switchconnected across a grounded conductor and a ground conduit; and

Figure 2 shows a modification wherein the temperature sensitive switchis connected across the conductors from the power source.

Description of Figure 1 The system is generally energized from a powersource, not shown, through a pair of conductors and conductors 16 and17. The conductors are shown to be shielded by a conduit 20.

The secondary 15 of transformer 13 is connected into an oscillatorcircuit 21 which is described and claimed in a Wannamaker application694,401 filed August 31, 1946, now Patent 2,564,937, issued August 21,1951, and assigned to the assignee of the present invention.

The oscillator circuit 21 includes an oscillator tube 22 and anamplifier tube 23. The oscillator tube 22 has an anode 24, a controlelectrode 25, a cathode 26, and a cathode heater 27. Amplifier tube 23has an anode 30, a control electrode 31, a cathode 32, and a cathodeheater 33. Heaters 27 and 33 are energized from any satisfactory source,not shown.

A tank circuit 34 having an inductance coil 35 and a capacitor 36 isconnected to the anode 24 of the oscillator tube 22. An inductance coil37 is connected to the cathode 26 of the oscillator tube 22. A thirdinductance coil 43 inductively coupled inductance coils 35 and 37.

Oscillator tube 22 is energized from transformer secondary 15 by thefollowing circuit: from the upper end of transformer secondary 15through a relay winding 40 of relay 41, conductor 42, tank circuit 34,anode 24 and cathode 26 of oscillator tube 22, inductance coil 37 andshielded conductor 17 to the lower terminal of transformer secondary 15.

Inductance coil 43 is connected in series with a capacitor 44 acrossconductor 17 and ground conduit 20 by the following circuit: fromconduit 20, through conductor 45, capacitor 44, conductor 46, inductancecoil 43, and conductor 47 to the conductor 17.

Control electrode 25 of the oscillator tube is connected to the groundedconductor 17 through a bias resistor 50 having a bypass capacitor 51connected in parallel therewith. The control electrode is also connectedto the control electrode 31 of the amplifier tube 23 through a conductor53.

Amplifier tube 23 is energized by transformer secondary 15 by thefollowing circuit: from the upper terminal of transformer secondary 15,through relay winding 40, conductor 42, anode 30 and cathode 32 ofamplifier tube 23, bias resistor 54 and grounded conductor 1.7 to thelower terminal of transformer secondary 15.

A capacitor 52 is connected between conductor 42 and grounded conductor17 and is of a size which acts as a high frequency short circuit toground.

Relay winding 40 has a capacitor 55 in parallel therewith to permitenergization of the winding over the entire cycle when the relayenergizing circuit is operative. The winding operates upon a movablecontact 56 which makes and breaks connection with a fixed contact 57.Movable contact 56 is biased by a spring 60 which is held fixed at oneend as at 61.

When contacts 56 and 57 are closed a circuit is cornpleted through awarning light 62 which is connected to a battery 64 by the followingcircuit: from battery 64. through light 62, conductor 65, fixed contact57, movable contact 56, and conductor 66 back to the battery.

A temperature-sensitive switch 67 is connected in series with acapacitor 70 across ground conductor 17 and ground conduit 20 by thefollowing circuit: from grounded conductor 17, through conductor 71,temper ature-sensitive switch 67, capacitor 70, and conductor 72 4 toconduit 20.

in the room where it is desired to detect for fire.

the room and the junction where the two are connected together is greatenough to load coil 43 to such an extent as to load tank circuit 34 andinductance coil 37 so that the oscillator circuit does not oscillate.

The values of the components given below have been tried and worksatisfactorily in the circuit but obviously other values may be used andtherefore these values are given only as an example and are not intendedto be limiting on the disclosure.

Component: Value Tube 22 Tube 23 i 7N7 Resistor 50 kilohms 470 Resistor54 ohms 500 Capacitor 36 micromicrofarads 500 Capacitor 44 microfarad .5Capacitor 51 do 0.01 Capacitor 52 do 0.005 Capacitor 55 do 4.0 Capacitor70 do 1.0

Operation of figure 1 The oscillatory condition of the circuit isdetermined by the loading on coil 43. When the current flows throughcoils 35 and 37 a voltage is induced in coil 43. The loading, orimpedance, connected to coil 43 then determines the current flow throughthe coil. This current induces a voltage back into the coils 35 and 37.When the loading on coil 43 is heavy the voltages induced back intocoils 35 and 37 are insufficient to cause the circuit to oscillate.Conversely, a light loading on coil 43 results in a large currentthrough the coil 43 and large voltages induced in coils 35 and 37 tobring about oscillation.

When the circuit is oscillating the anode 24 of oscillator tube 22 drawsvery little current, which current is far less than that necessary toenergize relay winding 40 to open contacts 56 and 57 against the actionof spring 60. The high frequency alternating voltage induced in coil 37is fed back to control electrode 25 through capacitor 51 which acts as ashort circuit for high frequency potentials to efiectively place controlelectrode 25 at the same potential as the lower terminal of coil 37.

Under oscillatory conditions the control electrode of an oscillator tubedraws appreciable grid current. This current is a pulsating directcurrent with the electron flow from cathode to control electrode onalternate half cycles.

As can be seen in the drawing, a circuit is completed between thecontrol electrode and cathode through resistor 50, with capacitor 51 inparallel, conductor 17, and coil 37 to the cathode. The flow of currentthrough this circuit under oscillatory conditions produces a voltagedrop across resistor 50 and thus drives the control electrode 25negative with respect to grounded conductor 17. During the half cyclesthe current flows from the control electrode to the cathode, capacitor51 is charged up and on the succeeding half cycles when no grid currentis drawn by the control electrode capacitor 51 discharges throughresistor 50. This charging and discharging maintains the controlelectrode 25 negative with respect to cathode 26.

On the half cycles that the control electrode draws grid current thereis also a current flow through coil 37 which causes cathode 26 to bepositive with respect to grounded conductor 17. During the half cyclethat no grid current is drawn the cathode is at ground potential but, ashas been seen, the control electrode 25 is negative with respect to thegrounded conductor during the entire cylcle and thus is always negativewith respect to the catho e.

Control electrode 31 of amplifier tube 23 is directly connected tocontrol electrode 25 of oscillator tube 22. Thus, with control electrode25 having a negative direct voltage impressed thereon with respect toground potential, the control electrode 31 is also negative with respectto ground potential and does not conduct.

When the oscillator tube is in a non-oscillatory condition there is verylittle grid current drawn by the control electrode and thus there islittle voltage drop across resistor With the control electrode notdriven to as great a negative potential as it is under oscillatoryconditions, the tube draws appreciable current.

Because control electrode 31 is directly connected to set, the switch 67is open and the inductance coil 43 is loaded by the entire impedance ofthe grounded conductor 17 and the conduit 20 between their connection tothe coil 43 and the junction where they are directly connected together.With the circuit constants properly adjusted this impedance loads thecoil 43 to such an extent that the voltage induced back into coils 35and 37 is too small to permit the oscillator circuit to oscillate. As aresult both the oscillator tube 22 and the amplifier tube 23 conduct andtogether operate to energize relay winding 40.

When the relay winding 40 is energized it operates upon movable contact56 to pull against spring 69 and' holds the connection between contacts56 and 5'7 open so that warning light 62 is not energized.

When the temperature becomes so high as to indicate a fire, or dangerouscondition, the bimetal switch. 67 closes to effectively short out mostof the impedance between' the grounded conductor 17 and ground conduit20 and decrease the loading on coil 4-3. When this condition occurs theloading on coil 43 is light enough to permit the oscillator circuit tooscillate. When the oscillator tube is oscillating there is, as has beenpreviously stated, very little plate current drawn by the oscillatortube. At the same time amplifier tube 23 is placed in a nouconductivecondition due to the negative voltage placed on control electrode 31with respect to ground. As a result, relay winding 40 is deenergizedpermitting spring 60 to pull movable contact 56 against contact 57 andcompleting the energizing circuit to the warning light 62 to give analarm. Obviously, a bell could be placed into the circuit to beenergized to give an audible alarm. Further, both a bell and light couldbe used together to give both an audible and a visual alarm.

In the event that something should go wrong with the oscillator oramplifier circuit such as a broken con nection or a defective tube theenergization of relay winding 40 is insutficient to hold the contacts inthe open position and the connection between contact 56 and 57 is closedto cause warning light 62 to give an indication that something is wrong.Also if there should be a power ponents or the power source the circuitfails safe.

Description of Figure 2 The circuit of Figure 2 differs from that ofFigure 1 only in the manner in which the flame sensing circuit isconnected to the power system. Corresponding parts in the two circuitshave like references. In the circuit shown in Figure 2 the inductancecoil 43 is connected in series with capacitor 44 across power conductorsiii and 11 by the following circuit: from conductor 10 through conductor16, capacitor 44, conductor 46, coil 43, and conductors 45 and 17 topower conductor Ill. Temperature sensitive switch 67 in series withcapacitor 70 is connected across power conductors 10 and 11 by thefollowing circuit: from conductor 10 through conductor 16, capacitor 70,temperature sensitive switch 67, and conductors 71 and 17 to powerconductor 11.

Operation of Figure 2 In the circuit of Figure 2 the inductance coil 43is loaded by the irnpedance looking back to the power source. In aspecific example this impedance of the power lines is of the order of 8ohms and, with the proper circuit constants, loads the oscillatorcircuit sufficiently to prevent oscillation. With the circuit notoscillating, relay winding 40 is energized to hold the en ergizingcircuit to warning light 62 open and prevent energization of the light.

When the temperature reaches such a levei that the temperature sensitiveswitch 67 is closed the impedance across coil 43 is reduced fromapproximately eight ohms to the order of three ohms. This loading issufficiently light, with the proper circuit constants, to permit theoscillator circuit to oscillate and cause relay winding 40 to bedeenergized. When the winding is deenergized contacts 56 and 57 closedue to the action of spring 60, and warning light 62 is energized. Itis, of course, understood that there are other values of power lineloadings and that loadings other than three ohms by the switch 67 may beused.

' With the oscillator circuit, the loading coil, and the temperaturesensitive switch all being energized across the power line it ispossible to connect all of them to conductors 16 and 17 which could beconnected into a plug for insertion into a wall socket. Thus a simpleand easy installation of this system can be made which would take only amatter of several seconds. If desired, the oscillator and loading coilcould be connected to one wall plug with the temperature sensitiveswitch connected to a second wall plug, each of which would be coupledinto a separate wall socket, the only provision being that theconductors energizing both wall sockets be on the same power line sothat closing of the contacts of the temperature sensitive switch wouldshort out the impedance of the power lines and reduce the loading on thecoupling coils.

The following values differ from their counterparts in Figure 1 and aregiven merely as an example of values which may be used to givesatisfactory operation. The components having the same values as theircounterparts are not repeated.

Component: Value Tube 22 Tube 23} 12AU7 Though I have shown preferredmodifications of my fire alarm system it is obvious that changes andmodification will be apparent to those skilled in the art and it is tobe understood that I limit myself only to the extent of the appendedclaims.

I claim as my invention:

1. Detecting apparatus using existing conventional electric power cablesfor a fire alarm system having in combination: an oscillator circuitcomprising an electron tube having an anode, a control electrode and acathode, a first inductance coil between said control electrode andcathode, a second inductance coil between said anode and controlelectrode; means energizing said circuit from a voltage source throughexisting electric power conductors one of which is grounded; a thirdinductance coil inductively coupling said first and second coils; meansconnecting the third coil between the grounded conductor and a groundconduit about the power conductors such that the impedance of theconductor and conduit loads the third coil to prevent the circuit fromoscillating; and a temperature sensitive switch connected across thegrounded conductor and conduit which, when said switch is actuated,reduces the loading on the third coil and causes the circuit tooscillate.

2. In combination: an oscillator circuit comprising an electron tubehaving an anode, a control electrode and a cathode, a first inductancecoil between said control electrode and said cathode, a secondinductance coil between said anode and said cathode; means energizingsaid circuit from existing conventional conductors from a voltagesource, one of the conductors being grounded; a third inductance coilinductively coupling said first and second coils; means connecting thethird coil between the grounded conductor and a ground conduit about thepower conductors such that the impedance of the conductor and conduitloads the third coil to prevent the circuit from oscillating; and aswitch connected across the third coil which, when said switch isactuated, reduces the loading on the third coil and causes the circuitto oscillate.

3. In combination: an oscillator circuit, comprising at least aninductance coil connected across a conductor forming part of an existingconventional power supply system and a conduit about the conductor, theconduit also being a conductor; means connecting the conductor andconduit to each other at a distant point such that the impedance of theconductor and conduit loads the inductance coil to such an extent as toprevent the oscillator circuit from oscillating; a switch; and means soconnecting said switch across the conductor and conduit at a pointremote from said inductance coil that, when said switch is actuated, theloading on the coil is reduced and the circuit oscillates.

4. Detecting apparatus using existing conventional electric power cablesfor a fire alarm system comprising in combination: an oscillator circuitcomprising an electron tube having an anode, a control electrode and acathode, a first inductance coil between said control electrode and saidcathode, a second inductance coil between said anode and said cathode,and a third inductance coil inductively coupling said first and secondcoils; means energizing said circuit from a pair of existing conductorsfrom a voltage source; means connecting said third coil in series with acapacitor between one of the pair of conductors and a grounded shieldingconduit about the pair of conductors, the one of the pair of conductorsbeing grounded, such that the impedance of the one of the pair ofconductors and the conduit loads the third coil and prevents the circuitfrom oscillating; and a temperature sensitive switch connected in serieswith a capacitor across the one conductor and the conduit at a pointremote from the connections of said third coil and capacitor in serieswith said third coil across the one conductor and conduit, which, upon asufiicient rise in temperature closing said switch, shorts out a portionof the impedance of the one conductor and conduit loading the third coiland causes the circuit to oscillate.

5. Detecting apparatus using existing conventional electric power cablesfor a fire alarm system comprising in combination: an oscillator circuitcomprising at least an inductance coil connected across a groundedconductor and a grounded shielding conduit about the conductor formingpart of an existing conventional electric power cable, the conductor andconduit being connected to each other at a distant point such that theimpedance of the conductor and conduit loads the inductance coil andprevents the oscillator circuit from oscillating; a temperaturesensitive switch; and means so connecting said switch across theconductor and conduit at a point remote from the connections of saidcoil to the conductor and conduit that, upon a suificient rise intemperature closing said switch, a portion of the impedance loading saidcoil is shorted out and causes the circuit to oscillate.

6. Detecting apparatus for a fire alarm system for use in a buildinghaving a conventional shielded cable for supplying electrical power toelectrical equipment in various areas thereof, said apparatus comprisingin combination: an oscillator comprising an electron tube and a tunedcircuit including a first inductance coil, said oscillator being adaptedto be connected to the voltage supply conductors of the shielded cableto be energized thereby, the shield of the cable being a furtherconductor, and said oscillator being adapted to be located in oneportion of the building and comprising an indicator for indicating whenthe state of oscillation of said oscillator is changed; a couplingcircuit for said oscillator comprising a second inductance coilinductively coupled to said previously named coil and having theterminals thereof connected to two of the conductors of the cable, saidcoupling circuit being located in the same area as said oscillator; anda fire responsive switching mechanism adapted to be located in an arearemote from said oscillator and said coupling circuit and into whicharea the cable extends for supplying power to electrically operatedapparatus within that area, said switching mechanism being connectedbetween the same two conductors as said second coil so that upon closureof said switching mechanism a circuit is completed, including saidsecond inductance coil, the two conductors to which the second coil ands /itching mechanism are connected and said switching iechanism,shorting out a portion of the impedance of the two conductors loadingsaid second coil.

7. Detecting apparatus for a fire alarm system for use in a buildinghaving a conventional shielded cable for supplying electrical power toelectrical equipment in various areas thereof, said apparatus comprisingin combination: an oscillator comprising an electron tube and a tunedcircuit including a first inductance coil, said oscillator being adaptedto be connected to the conductors of the shielded cable to be energizedthereby and said oscillator being adapted to be located in one portionof the building and comprising an indicator for indicating When thestate of oscillation of said oscillator is cha d: a coupling circuit forsaid oscillator comprising a second inductance coil inductively coupledto said previously named coil and having the terminals thereof connected7 to .a first of the conductors of the cable and the shield thereof saidcoupling circuit being located in the same area as said oscillator; anda fire responsive switching mechanism adapted to be located in an arearemote from said oscillator and said coupling circuit and into whicharea the cable extends for supplying power to electrically operatedapparatus within that area, said switching mechanism being connectedbetween the first conductor of the cable and the shield thereof so thatupon closure of said switching mechanism a circuit is completed,including said second inductance coil, the first conductor, the shieldandsaid fire responsive switching mechanism, shorting out a portion ofthe impedance of the first conductor of the cable and the shield thereofloading said second coil.

8. A fire detecting device using an existing electr cal power systemboth as a power source and an electrical load for the device,comprising; an oscillator circuit having at least one inductance coil,power circuit means including conductors adapted to be connected to anelectrical power system to continuously supply power to said oscillator,load circuit means including said coil and conductors adapted to beconnected to the electrical power system to normally load said coil suchas to prevent the circuit from oscillating; and a temperature responsiveswitch connected across said load circuit means which, when said switchis actuated, reduces the loading on said coil and causes the circuit tooscillate.

9. A fire detecting device using an existing electrical wiring systemboth as a power source and as an electrical load for the device,comprising; an oscillator circuit cornprising an electron tube having ananode, a control electrode and a cathode, a first inductance coilbetween said control electrode and said cathode, and a second inductancecoil between said anode and said cathode, said first and second coilsbeing inductively coupled; power circuit means including conductorsadapted to be connected to an existing wiring system to continuouslysupply power to said oscillator circuit, load circuit means arranged toinfluence the inductive coupling of said first and second coils andincluding conductors adapted to be connected to the existing wiringsystem to normally pre- Vent said oscillator from oscillating, and atemperature reponsive switch connected to said load circuit means tocause said oscillator to oscillate when said switch is actuated.

10. An electronic fire detecting device using an existing electricalwiring system both as a power source and as an electrical load for theelectronic device, comprising; an oscillator circuit comprising, anelectron tube having an anode, a control electrode and a cathode, afirst inductance coil electrically connected between said cathode andsaid control electrode, a second inductance coil electrically connectedbetween said anode and said cathode, and a third inductance coilinductively coupling said first and second coils; power circuit meansincluding conductors adapted to be connected to an existing electricalWiring system to continuously supply power to said oscillator circuit,loading circuit means including said third coil and conductors adaptedto be connected to the existing wiring system to connect said third coilto the existing wiring system to load said third coil and prevent saidoscillator circuit from oscillating, and a temperature responsive switchconnected across said third coil to reduce the loading on said thirdcoil when said switch is actuated to cause said oscillator circuit tooscillate.

11. A condition sensing device using an existing electrical power systemboth as a power source and as an electrical load for the device,comprising; an oscillator circuit comprising an electron tube having ananode, a control electrode and a cathode, a first inductance coilbetween said control electrode and said cathode, a second inductancecoil between said anode and said cathode, and a third inductance coilinductively coupling said first and second coils; power circuit meansincluding conductors adapted to be connected to an existing power systemto continuously supply power to said oscillator circuit, load circuitmeans including said third coil and conductors adapted to be connectedto the existing power system, said load circuit means being adapted toload said third coil to thereby prevent said oscillator circuit fromoscil lating; and a condition sensitive switch connected across thethird coil which, when actuated reduces the loading on the third coiland causes the circuit to oscillate.

12. A fire detecting device using an existing electrical power cable asa source of power and as an electrical load, comprising, an oscillatorcircuit comprising an electron tube having an anode, a control electrodeand a cathode, a first inductance coil between said control electrodeand said cathode, a second inductance coil between said anode and saidcathode; power circuit means including conductors adapted to becontinuously connected to power conductors of an existing electricalpower cable; a third inductance coil inductively coupling said first andsecond coils;.load circuit means including said third coil andconductors adapted to be connected to the power conductors of theexisting electrical power cable to connect said third coil across thepower conductors such that the impedance of the power conductors loadsthe third coil to prevent said oscillator circuit from oscillating; anda temperature responsive switch connected across the power conductorswhich, when said switch is actuated, reduces the loading on the thirdcoil and causes the circuit to oscillate.

13. A condition sensing device using an existing electrical power cableas a source of power and as an electrical load, comprising; anoscillator circuit comprising at least an inductance coil; power circuitmeans including said inductance coil and conductors adapted to beconnected to a pair of power conductors of an existing electrical cableto connect the inductance coil across the power conductors such that theimpedance of the power conductors loads the inductance coil to such anextent as to prevent the oscillator circuit from oscillating; acondition sensing switch; and means including further conductors adaptedto be connected to the power conductors at 'a point remote from saidinductance coil to so connect said switch that when said switch isactuated, the loading on the coil is reduced and said oscillator circuitoscillates.

14. in combination: an oscillator circuit, comprising at least aninductance coil connected across a conductor forming part of an existingconventional power supply system and a conduit about the conductor, theconduit also being a conductor, said oscillator having a first and asecond condition of operation; means connecting the conductor andconduit to each other at a distant point such that the impedance of theconductor and conduit load the inductance coil to such an extent as tocause said oscillator to assume said first condition of operating; aswitch; and means so connecting said switch across the conductor andconduit at a point remote from said inductance coil that, when saidswitch is actuated, the loading on the coil is altered and theoscillator assumes said second condition of operation.

15. A condition sensing device using an existing electrical power systemboth as a power source and as an elec trical load for the device,comprising: an oscillator circuit having at least one inductance coil,said oscillator having a first and a second condition of operation,power circuit means including conductors adapted to be connected to anelectrical power system to continuously supply power to said oscillator,load circuit means in cluding said coil and conductors adapted to beconnected to the electrical power system to load saidcoil such as tocause said first condition of operation of said oscillator circuit; anda temperature responsive switch connected across said load circuit meanswhich, when said switch is actuated, alters the loading on said coil andcauses said second condition of operation of said oscillator circuit.

16. An electronic fire detecting device using an existing electricalwiring system both as a power source and as an electrical load for theelectronic device, comprising: an oscillator circuit comprising, anelectron tube having an anode, a control electrode and a cathode, afirst inductance coil electrically connected between said cathode andsaid control electrode, a second inductance coil electri cally connectedbetween said anode and said cathode, and a third inductance coilinductively coupling said first and second coils; said oscillatorcircuit having a first and a second condition of operation dependingupon the loading of said third coil; power circuit means includingconductors adapted to be connected to an existing electrical wiringsystem to continuously supply power to said oscillator circuit, loadingcircuit means including conductors adapted to be connected to theexisting wiring system to connect said third coil to the existing wiringsystem to load said coil and cause said first condition of operation ofsaid oscillator circuit, and a temperature responsive switch connectedacross said third coil to alter the loading on said third coil when saidswitch is actuated to thereby cause said second condition of operationof said oscillator Number Name Date circuit. 2,169,977 MacDonald Aug.15, 1939 2,355,606 Shannon Aug. 15, 1944 References Cited in the file ofthis patent 5 gerlrly Akpr 26, 1949 ic pr 25, 1950 UNITED STATES PATENTS2,566,121 Decker Aug. 28, 1951 Number Name Date 2,567,908 Levy Sept. 11,1951 1,838,084 Drake Dec. 29, 1931 2,584,728 Michel Feb. 5, 19522,041,114 Carini May 19, 1936

